Recently, interest in energy storing technologies is gradually increasing. As application areas expand to energies for mobile phones, camcorders and notebook PCs, and further, to electric vehicles, efforts for research and development on electrochemical elements are gradually materializing. In this aspect, electrochemical element is the area gathering the most attention, and most of all, developing secondary batteries capable of charging and discharging is becoming the focus of attention, and recently, in developing such batteries, research and development to design new electrodes and batteries in order to improve capacity density and specific energy is under way.
As a positive electrode active material of a lithium secondary battery, lithium-containing cobalt oxide (LiCoO2) is mostly used, and besides that, lithium-containing manganese oxides such LiMnO2 of crystalline structures on layers and LiMn2O4 of a spinel crystalline structure and the like, and lithium-containing nickel oxide (LiNiO2) are also being considered for usage.
Of the positive electrode active materials, LiCoO2 has excellent overall properties such as excellent cycle characteristics and the like, and is thus in wide use, but it is expensive due to limitation of resources of cobalt as a raw material. Lithium manganese oxides such as LiMnO2, LiMn2O4 and the like have advantages that they use manganese that is abundant in resources and that is environmentally-friendly, and thus such lithium manganese oxides draw much interest as positive electrode active materials that could replace LiCoO2. However, these lithium manganese oxides have disadvantages that they have small capacity and that they have bad cycle characteristics and the like.
Further, lithium nickel oxides such as LiNiO2 and the like are inexpensive compared to the cobalt oxides, while showing high discharge capacity when charged to 4.25V, and thus the reversible capacity of doped LiNiO2 comes close to about 200 mAh/g, that exceeds the capacity of LiCoO2 (about 153 mAh/g). Therefore, despites its slightly low average discharge voltage and volumetric density, a commercialized battery including LiNiO2 positive electrode active material has improved energy density, and thus, in recently days, research on such nickel positive electrode active materials is actively under way in order to develop high capacity batteries.
Therefore, a lot of conventional technologies focus on improving the characteristics of LiNiO2 positive electrode active materials and on preparation processes of LiNiO2, and a lithium transition metal oxide having a form where a portion of nickel is substituted for another transition metal such as Co, Mn and the like was proposed. However, problems such as the high production cost of LiNiO2 positive electrode active material, swelling caused by generation of gas in a battery, low chemical stability, and high pH and the like are not being solved sufficiently.
Thus, some prior art documents applied materials such as LiF, Li2SO4, Li3PO4 and the like on surfaces of lithium nickel-manganese-cobalt oxides in attempts to improve the performance of batteries, but in such cases, the aforementioned materials were disposed on only the surfaces of the lithium nickel-manganese-cobalt oxides, causing a problem of not only placing limitations to exerting a desired extent of effects, but also requiring a separate process for applying the aforementioned materials on the surfaces of the lithium nickel-manganese-cobalt oxides.
However, despite such various attempts, a lithium composite transition metal oxide having satisfactory performance has yet to be developed.